Canonically, the sexual dimorphism in the brain develops perinatally, with adult sexuality emerging due to the activating effects of pubescent sexual hormones. This concept does not readily explain why children have a gender identity and exhibit sex-stereotypic behaviours. These phenomena could be explained if some aspects of the sexual brain networks have childhood forms, which are transformed at puberty to generate adult sexuality. The bed nucleus of stria terminalis (BNST) is a dimorphic nucleus that is sex-reversed in transsexuals but not homosexuals. We report here that the principal nucleus of the BNST (BNSTp) of mice has developmental and adult forms that are differentially regulated. In 20-day-old prepubescent mice, the male bias in the principal nucleus of the BNST (BNSTp) was moderate (360 +/- 6 vs 288 +/- 12 calbindin(+ve) neurons, p < 0.0001), and absent in mice that lacked a gonadal hormone, AMH. After 20 days, the number of BNSTp neurons increased in the male mice by 25% (p < 0.0001) and decreased in female mice by 15% (p = 0.0012), independent of AMH. Adult male AMH-deficient mice had a normal preference for sniffing female pheromones (soiled bedding), but exhibited a relative disinterest in both male and female pheromones. This suggests that male mice require AMH to undergo normal social development. The reported observations provide a rationale for examining AMH levels in children with gender identity disorders and disorders of socialization that involve a male bias.